Polymer modified asphalt for paving applications and methods for preparing various polymer modified asphalt compositions are well known in the art. Various polymers, including diene elastomers such as polybutadiene, EPDM, EPR, and styrenic block copolymers (SBCs) such as styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) block copolymers, are commonly combined with bitumen to improve the performance of the bitumen. See, for example, U.S. Pat. No. 5,190,998 and U.S. Pat. No. 6,150,439. It is well known in the art to utilize styrenic block copolymers to improve the performance of bitumen. The degree to which property improvements are realized depends upon the compatibility of the block copolymer and the bitumen. Highly compatible or compatibilized polymers are most effective in providing property improvements. Over the years, researchers have developed a range of techniques to improve the compatibility of these types of polymers with bitumens.
In addition, there are improvements associated with the use of styrenic block copolymers which the bitumen paving industry would like to see. These improvements include improved fatigue performance, greater resistance to permanent deformations, greater resistance to thermal cracking, greater resistance to induced cracking, and economic savings.
Bituminous compositions containing block copolymer compositions having a high vinyl content or block copolymer compositions having a high vinyl content and a low diblock content have been used in an attempt to provide bituminous compositions with better properties. See, for example, U.S. Pat. No. 4,530,652, U.S. Pat. No. 5,798,401, U.S. Pat. No. 5,854,335, and U.S. Pat. No. 6,508,875. Compositions high in diblock have also been used to prepare bituminous compositions in an effort to provide rapid and easy mixing and to improve dispersion. See, for example, U.S. Patent Publication No. 2005/0137295 and U.S. Patent Publication No. 2005/0004273. These earlier formulations solve the specific problems with respect to the wearing course, but they are inadequate and do not offer any substantial improvement in base course applications.
U.S. Patent Publication No. 2007/0112102, which is incorporated herein by reference, discloses a process for preparing a bituminous binder composition. The publication discloses the addition of a block copolymer composition to a bitumen component for a wearing course on a roadway. The wearing course of a roadway is the top layer that automobiles drive upon, and the wearing course is also the layer that is replaced during road re-paving. This publication fails to disclose a suitable bituminous composition for base course applications.
Bitumen is used as a binder in road asphalt mixtures, and has developed continually to meet ever increasing performance demands from road building constructors. In general, bitumen performs well in road asphalt, but increasingly heavy traffic loads have led to the premature wear of many roads through rutting, raveling (e.g., loss of aggregate material), cracking of the surface, and bottom-up cracking.
Roadway construction is generally comprised of three layers. The first layer placed on grade is a sub-course of granular material or aggregate. The second layer, which is placed upon the first layer, is a base course. Optionally, there is a binder course located between the base course and the wearing course that provides a smooth surface upon the base course. The base course is the thickest and most expensive layer to apply in the construction of roadways. In the present invention, the base course is a polymer modified bituminous mixture that may have a thickness of between about 25.4 mm to about 400 mm. However, the thickness may vary and largely depends upon the projected traffic density and loading. The final or top layer is the wearing course. The wearing course is the layer automobiles drive upon and are subject to the most wear and tear. When road surfaces are replaced, the wearing course is the layer that is removed and replaced between ten or fifteen years after construction. The base course is generally not replaced, but may be replaced when rutting or cracking has occurred. Instead, the full depth reconstruction must occur if the base course fails to adequately carry the traffic loads. The base course is subject to adverse conditions like the wearing course, and there is a need for a base course that has an improved fatigue performance, greater resistance to permanent deformations, and resulting economic savings.
The present invention discloses a coupled block copolymer with a low molecular weight. The present invention is particularly useful for preparing polymer modified bituminous binder compositions to be used for road/paving applications of all sorts, and in particular for a base course. By utilizing the present invention, it is possible to prepare a polymer modified bituminous binder composition that has an improved fatigue performance, greater resistance to permanent deformations, and economic savings. Furthermore, the present invention discloses a polymer that is more compatable with hard base course bitumens than standard polymers.
An ideal asphalt mixture should be capable of maintaining its stiffness characteristics throughout a range of design temperatures. At low temperatures, the asphalt mixture is stiff and prone to cracking. On the other hand, at high temperatures, the asphalt mixture becomes soft and prone to load induced inelastic deformations. In addition, the ability of the binder to bond with the aggregates is diminished over time, allowing raveling to occur. The utilization of polymer modified mixes in the asphalt has resulted in reducing the stiffness of the asphalt mix at low temperatures, and increasing the stiffness of the asphaltic mix at high temperatures. The use of a polymer modified mix in the base course of asphaltic paving applications has displayed improved fatigue performance, greater resistance to permanent deformations, and economic aspects.